1. Field of the Invention
The invention relates to magnetically tuned resonant circuits.
2. Description of Related Art
A magnetically tuned resonator is a magnetic insulator which resonates at a microwave frequency when placed in a magnetic field. If the resonator is spherical, the frequency of resonance is related only to the strength of the magnetic field and not to the radius of the sphere. Magnetically tuned resonators in the spherical form usually have a radius between 0.5 mm and 1.0 mm, and in the most common case are made of either single-crystal yttrium iron garnet or gallium-substituted yttrium iron garnet. A number of other magnetic materials may also be used. All of these materials are referred to herein as YIG. General background material on YIGs and YIG circuits can be found in J. Helszajn, "YIG Resonators and Filters," John Wiley & Sons (New York: 1985), incorporated by reference herein.
YIG resonators are used in several different types of microwave circuits, including filters, limiters and oscillators. The present invention, while usable with certain other types of microwave circuits, is useful primarily in tuned oscillator circuits.
In a generalized oscillator circuit, a reactive component is coupled to an active device which incorporates feedback. Feedback may be either series or parallel. For example, if the active device is an FET, then common source, common gate, and common drain configurations are possible, all of which may incorporate either series or parallel feedback. These configurations are shown at page 193 of Helszajn. In a YIG oscillator, a YIG sphere is used as a reactive component, and it is placed in a magnetic field to set its resonant frequency. As used herein, a reactive component need not be purely reactive; it can include some resistance as well.
For a tunable YIG oscillator, the YIG sphere is placed in the air gap of an electromagnet, and the current applied to the windings is varied as desired in order to obtain the desired frequency of oscillation. The electromagnet usually has a re-entrant cylindrical shape comprising a closed cylindrical magnetic shell having a pole piece extending inwardly from one end. A second pole piece may extend inwardly from the opposite end of the shell toward the first pole piece, but this is not essential. The end of the first pole piece defines a first pole surface, and the end of the second pole piece (or if there is no second pole piece then the other end of the magnetic shell itself) constitutes a second pole surface. The winding for the electromagnet is wrapped around at least one of the pole pieces. The first pole surface has a circular shape with a radius slightly larger than that of the YIG sphere, in order to ensure that the sphere is magnetized by a uniform magnetic field. The two pole surfaces are oriented parallel to each other for the same reason. The pole piece is usually cylindrical in shape, and tapers near its end to the size of the circular pole surface.
In the past, YIG oscillators have employed either an FET or a bipolar transistor as the active device coupled to the YIG resonator. FETs can operate to much higher frequencies than bipolar transistors can, but bipolar transistors have significantly better noise characteristics. Thus, if a designer needed a YIG oscillator tunable within a lower frequency range, for example 2-8 GHz, a bipolar transistor-based oscillator would be chosen. But if an oscillator tunable within a higher frequency range, for example 8-20 GHz, was desired, an FET-based oscillator would be chosen instead. No single broadband device has been available which can be tuned to frequencies with both the bipolar and FET microwave frequency ranges. Attempts have been made to increase the high frequency limit of bipolar transistor-based YIG oscillators by increasing the high frequency limit of the transistors, but these transistors have also tended to have higher minimum frequencies of operation. See, for example, Leung et al, "A 0.5 .mu.m Silicon Bipolar Transistor for Low-Phase Noise Oscillator Applications Up to 20 GHz," 1985 IEEE MTT-S Digest, pp. 383-386, and Leung et al, "Downsized Bipolar Fires 20 GHz Oscillator," Microwaves & RF (September 1985), pp. 163-168. Similarly, though FET-based YIG oscillator circuits can be designed to operate at either low or high frequencies, it is difficult to design a single circuit which can be tuned over the entire broadband. Thus, there is a need for broadband tuned YIG oscillators in a single device.
According to the parent application, a broadband YIG tuned oscillator may be obtained by combining two YIG oscillator circuits in a single housing with both YIG spheres being disposed in the same magnetic structure. The first circuit may be an FET-based oscillator circuit while the second circuit is a bipolar transistor-based oscillator circuit. The outputs of the two circuits are switchably coupled to an output terminal to enable the user to select the output of the bipolar transistor-based oscillator for lower frequencies and the output of the FET-based oscillator for higher frequencies. The current applied to the coil for the sole electromagnet determines the frequency output of both oscillator circuits.
While this arrangement works quite well, in certain implementations it may be necessary to operate one of the oscillators at an operating frequency close to an edge of its optimum range. This could generate some harmonic distortion when operated at that frequency which may exceed desired specifications. For example, the bipolar oscillator may be designed to operate within the range of 2-8.4 GHz, and the FET oscillator designed to operate within the range of 8.4-20 GHz. As the operating frequency of the FET oscillator is reduced toward its lower extreme, however, harmonic distortion tends to increase and may exceed the desired specification.
One possible solution to this problem might be to design the FET oscillator to operate with a larger frequency range. This may be difficult, especially if the oscillator must maintain its power output and frequency range even at higher temperatures. Both power output and maximum frequency tend to decrease as the temperature of an FET YIG oscillator increases.
Another solution might be to add a low-pass filter in the circuitry outside the multiple YIG oscillator shell. However, such filters typically are larger and have better performance specifications than truly necessary to reduce the small excess in harmonic distortion.